To cope with the growth in air traffic and improve the safety thereof, the aeronautical authorities represented by the ICAO (acronym standing for International Civil Aeronautical Organization) have defined precise recommendations relating to improvements in communication services within the framework of the future ATN (Aeronautical Telecommunication Network) civil aeronautical communication network and the implementation of future air traffic management or ATC (Air Traffic Control) applications. ATC communication systems are based on onboard/ground information exchanges, the aim of which is to ensure a secure and effective link between pilots and air traffic controllers.
Among the various applications specified by the ICAO (International Civil Aeronautical Organization) on the ATN network, one of them, the CPDLC (Controller Pilot Data Link Communication) allows the exchange of digital traffic control messages between the pilot and the air traffic control authority.
Moreover the large majority of aircraft are provided with a flight management computer (also called Flight Management or FM) whose job is to calculate the position and the attitude of the craft as well as to calculate and optimize the trajectory of the aircraft so as to follow the envisaged flight plan. This computer is parameterized according to the options chosen by each airline, notably as regards the measurement units to be used for the altitude and speed data.
The phraseology of the CPDLC messages exchanged with an air traffic control authority is standardized. For example, when the air traffic controller requests a change of altitude from the pilot, he does it with a CPDLC message of the type “CLIMB at LEVEL XXX” where XXX is a numerical data item which represents the desired altitude. This numerical data item can equally well be expressed by the controller in feet or in meters. The same holds for the data exchanges relating to a speed (expressed in kilometers per hour or in knots), a pressure (expressed in hectopascals or in inches of Mercury) or else a distance (expressed in nautical miles or in kilometers). The unit of the numerical data item is mentioned in the CPDLC message and is displayed on a screen of the flight deck. Nevertheless, the fact that it can differ from that parameterized in the flight management computer compels the pilot to verify the measurement unit of a numerical setpoint of a CPDLC message before being taken into account by the flight management system. Nevertheless, the communication of a numerical data item expressed in a unit which differs from those used by the flight management computer complicates the work of the pilot when the data item is transmitted to the computer. In some circumstance, this complication may give rise to errors in the value of the numerical data transmitted to the flight management computer and may entail extremely serious consequences for the life of the occupants of the aircraft.